ABSTRACT/PROJECT SUMMARY Background: Hybrid speciation?when hybrids become reproductively isolated from their parental species?is argued to be common in plants, but the importance of the process in animals is hotly debated. The difficulty of obtaining evidence for animal hybrid speciation has led to two camps in speciation research: those who state that hybrid speciation in animals is rare, and those who consider the process to be rampant. The answer is bound to lie somewhere in between these two extremes, but no experimental tests have been attempted. Broad, long-term objective: My research goal is to use carefully designed experiments to understand the importance of different mechanisms of speciation. Arguably, hybrid speciation remains the most controversial of all evolutionary process that generates new lineages. Drosophila is a premier animal system to study experimental speciation and in this proposal I leverage its power to assess the importance of hybrid speciation. Specific aims: The goal of this project is to understand how likely it is for new animal species to arise through hybridization. Aim 1 will produce hybrid swarms between 35 species pairs with different levels of genetic divergence and measure what proportion of replicates give rise to reproductively isolated hybrid species. Aim 2 will study what ecological and genetic factors facilitate hybrid speciation. Aim 3 will assess the genetic changes underlying these parallel experimental hybrid speciation events, and specifically test previously published hypotheses about the genetic causes of hybrid speciation. Method: Fruit flies from the genus Drosophila can be maintained under laboratory conditions, and have an unmatched arsenal of molecular, genetic, and genomic tools. This proposal presents an experimental evolution approach using 35 hybridizing species pairs of Drosophila to understand how frequently hybrid speciation occurs. I will then gather genome wide data and identify the alleles associated with hybrid speciation. Preliminary results for the experimental evolution and mapping components are both promising. Health-relatedness: Both the questions?how prevalent is speciation by hybridization?and our approach to answering them?a new model to detect admixture in hybrid genomes?have relevance for human health. First, hybridization between insect species might generate new disease vectors. Yet, the evidence for or against hybrid speciation remains scant. Second, the computational tools we have developed are flexible enough to be used in any organism and they provide an advantage over other existing methods. Impact: Hybrid speciation is one of the most difficult speciation processes to conclusively demonstrate, because it is often impossible to rule out alternative explanations. Experimental evolution provides a means to observe hybrid speciation without ambiguity. The results from this proposal can resolve the conflict regarding the prevalence of hybrid speciation. Importantly, the results will also reveal whether there is a particular level of genetic divergence that is most suitable for the evolution of hybrid species.